I’m sorry your bird is not feeling well. Since this came on “suddenly” it may be something like a bacterial infection that can be treated with antibiotics. Birds will hide signs of illness so once they do show symptoms, they have generally been sick for at least a few days. I would take her to an avian vet as soon as possible. It could be age related – 20 can be old for a cockatiel, but some live to be older. And generally age related slowing down is gradual. Without a vet exam, there is no way to be sure. I hope she feels better soon.